The known processes for producing cold and/or heat by means of an absorption cycle use a working fluid (F) and a solvent (S). The working fluid (F) is successively condensed while releasing heat, expanded, vaporized by taking heat from an external fluid, absorbed and then desorbed at a higher pressure.
U.S. Pat. No. 4,413,479 discloses an improvement to this cycle comprising the use of an auxiliary fluid (H) which vaporizes in admixture with fluid (F) and condenses while fluid (F) absorbs in solvent (S). Fluid (H), which is at least partially immiscible with the solution of fluid (F) in solvent (S), is separated from the rich solution of (F) in (S) obtained in the absorption step and recycled to the evaporation step.
Such a process is illustrated by the flowsheet of FIG. 1. Fluid (F) is discharged in vapor phase from the rectification zone R.sub.10 through line 11, then condensed in condensor E.sub.10, which is itself cooled by the external cooling stream 1-2. A fraction of the resultant liquid phase is fed through line 15 to the rectification zone and the remaining fraction is expanded through the expansion valve V.sub.10 and admixed with the liquid phase formed by fluid (H) supplied through line 13. The mixture of fluids (F) and (H) is vaporized in exchanger E.sub.11 by taking heat from an external fluid (lines 3-4) and the resultant vapor phase is fed through line 14 to the absorption zone A.sub.10 and admixed with the lean solution supplied through line 12, after expansion through the expansion valve V.sub.11.
During the absorption of fluid (F) and the condensation of fluid (H) in the absorption zone A.sub.10, the absorption heat as well as the condensation heat are transferred to an external fluid (circuit 7,8). At the output of the absorption zone, the two liquid phases formed on the one hand of a solution of fluid (F) in solvent (S) and, on the other hand, of fluid (H), are separated in the settling drum BD1. The solution of fluid (F) in solvent (S) obtained at the output of the settling drum BD1 is conveyed to the desorption column which comprises the rectification zone R.sub.10 and the stripping zone S.sub.10, through pump P.sub.10, line 9 and exchanger E.sub.12 wherein it is heated by heat exchange with the lean solution from the desorption column (line 10). In the bottom portion of the desorption column, the solution from the absorption step is heated and produces a vapor phase which is fed to the rectification zone R.sub.10 wherein the vapor phase is separated from the major part of solvent (S), driven along with the vapor phase, this rectification being unnecessary when no noticeable carrying of solvent occurs. The liquid phase of fluid (H) is withdrawn by pump P.sub.15 and recycled to the input of evaporator E.sub.11 through line 13.
The selection of fluid (H) is very important and determines the gain factor, defined as the ratio (F.sub.1 +F.sub.2)/F.sub.1 wherein F.sub.1 is the vaporization heat of fluid (F) and F.sub.2 the vaporization heat of fluid (H) involved in evaporator E.sub.11.
This gain factor increases with the proportion of fluid (H) vaporized in evaporator E.sub.11 and it has been shown in the above-mentioned patent that the use of a relatively volatile fluid (H) is advantageous, the volatility of fluid (H) being limited by the necessity of condensing fluid (H) during the absorption of (F) in (S).